Apple shares took the biggest single-day hit in four years on Wednesday, falling more than 6 percent on NASDAQ to $538.79.

The tumble took Apple's market cap down to $506.84bn, well below the dizzy heights of $705.07 billion reached in September this year. Yesterday saw an eye-watering $34.8 billion wiped off the total value of the company in one day.

Investors cited fears about competition in the mobile market and the threat from both Android and Nokia/Windows as factors in the fall.

But competition has always been strong and so it's likely to be factors from within Apple itself that are shifting perceptions. The iPad Mini hasn't been the wunderhit many were expecting, with warm but not raving reviews and very muted crowds at the launch of the device. The absence of any other big hitter devices on the horizon - a new Apple TV still seems far off - reinforces the view that Apple may be losing its innovative edge in the Tim Cook era.

On the financial side, Reuters reported unconfirmed rumours that that at least one major stock-clearing house was raising margin requirements on Apple stock trades.

The actions of a rogue trader who used 1.625 million Apple shares to pull a rogue deal worth $1 billion just before Apple's quarterly results came out, can't have helped matters either.

The dodgy deal was discovered when Apple's quarterly earnings on 25th October missed expectations, shares fell and the trader's employer took a massive hit on the trade.